1. Field of the Invention
This invention generally relates to an optical information recording and reproducing apparatus, and, in particular, to an optomagnetic (or magnetooptic) information recording and reproducing apparatus for optically recording or reproducing information on or from an optomagnetic recording medium, such as an optomagnetic disc, by utilizing the direction of polarization.
2. Description of the Prior Art
FIG. 9 illustrates a typical prior art optical pick-up capable for use in recording, reproducing or erasing information on or from an optomagnetic disc (not shown). The optical pick-up includes an objective lens 1 located facing an optomagnetic disc, an illumination optical system 2, a servo optical system 3 and an optomagnetic detection optical system 4, which are all mounted on a substrate 5. In the illumination optical system 2, laser light emitted from a semiconductor laser 6 is collimated by a coupling lens 7 and the thus collimated light is passed through beams-shaping splitters 8 and 9 to thereby define a light beam circular in shape. Then, this light beam is incident upon a polarization beam splitter 10 which serves to separate the incident light beam from a reflected light beam and which is provided with a polarizing surface 10a for allowing p waves to be transmitted at 100% and reflecting s waves at 2/3, so that only the s wave components are reflected by the polarizing surface 10a and the reflected light beam then passes through the objective lens 1 to be directed toward an optomagnetic disc (not shown).
When the light is reflected by the optomagnetic disc, it is rotated either in the positive or negative direction over a predetermined angle of polarization depending on the recording condition on the optomagnetic disc. Thus, the reflected light beam includes p waves and is directed toward beam splitters 10 and 11. Thus, the s waves reflected by the polarizing beam splitter 11 for separating a servo detecting system and an optomagnetic detection optical system travel through a detection lens 12 of the servo detection system 3 and a knife edge prism 13 and are received by a tracking light detector 14 and a focusing light detector 15. A tracking detection is carried out by a so-called push-pull method. The s waves transmitting through the polarization beam splitters 10 and 11 and the p waves produced by reflection have their planes of polarization rotated over 45.degree. when passing through a half wavelength plate 16 located next to the polarization beam splitter 11 in the optomagnetic detection optical system 4. Furthermore, p and s wave components are separated by a Wollaston prism 17 and then an image is formed on a light detector 19 by a detection lens 18 to thereby detect the direction of polarization, whereby the presence of absence of a recorded signal is detected.
However, in such a prior art structure, since tracking signal detection, focusing signal detection and optomagnetic signal detection are carried out by respective, separate optical systems, there must be provided a great number of optical components. In this case, there must be provided the half wavelength plate 16 and the Wollaston prism 17 as optical components for detecting the direction of polarization of reflected light in the optomagnetic detection optical system 4, and, thus, the optical system is rather complicated in structure. In particular, positioning must be carried out for the polarizing beam splitter 10, half wavelength plate 16 and Wollaston prism 17 individually. As a result, the number of steps in assembly increases and the stability tends to be impaired. Moreover, since there must be provided such a great number of optical components, the entire apparatus is rather heavy in weight and the access speed is rather slow.
In order to provide a high-speed access time in an optomagnetic disc apparatus, it is imperative to make an optical pick-up compact in size and light in weight. In this respect, there has previously been proposed an optical pick-up utilizing a high density diffraction grating as shown in FIG. 10, which was the subject of a Japanese Patent Application which has been assigned to the assignees of this application and thus hereby incorporated by reference. In the structure of FIG. 10, similarly with the case of FIG. 9, laser light emitted from a semiconductor laser 21 travels through a coupling lens 22, beam shaping splitters 23 and 24, a polarizing surface 25a of a polarization beam splitter 25 and an objective lens 26 and is focused onto an optomagnetic disc (not shown). The light reflected from the optomagnetic disc again travels through the objective lens 26 and the polarizing beam splitter 25 and then is separated from the incoming light beam to be directed toward a lens 27. Thereafter, the light enters a high density diffraction grating 28 which is inclined at a predetermined angle, where the incoming light is separated into transmitting light 29 of 0th order light and diffracted light 30 of 1st order light. The transmitting light 29 is received by a 4-division light-receiving device 31 for use in detecting a focusing signal; on the other hand, the diffracted light 30 is received by a 2-division light-receiving device 32 for use in detecting a tracking signal.
The high density diffraction grating 28 typically has such a polarization dependency characteristic as shown in FIG. 11. Thus, detection of an optomagnetic signal recorded on an optomagnetic disc is carried out as a difference between the outputs from the light-receiving devices 31 and 34. That is, with an angle of polarization denoted by alpha, the use rate of the 1st order light is approximately equal to sin.sup.2 alpha and the use rate of the 0th order light is approximately equal to cos.sup.2 alpha, so that a difference between the two is sin.sup.2 alpha-cos.sup.2 alpha. Detection of a focusing signal is carried out by an output from the 4-division light-receiving device 31 utilizing astigmatism which is produced by the lens 27 and the high density diffraction grating 28. Detection of a tracking signal is carried out by the 2-division light-receiving device 32 according to a push-pull method utilizing the 1st order light (diffracted light 30).
With this optical pick-up structure, the number of required components may be reduced and the overall structure may be made compact in size and light in weight as compared with the optical pick-up structure shown in FIG. 9. However, since use is made of the high density diffraction grating 28, the diffraction angle of the 1st order light would deviate significantly due to fluctuations in the wavelength of laser light from the semiconductor laser 21. For example, if a constant n of the high density diffraction grating 28 is equal to 1.5, when the wavelength abruptly changes by 2 nm, a light spot will be shifted over 0.15 mm at a point 30 mm away from the grating. Besides, since the two light-receiving devices 31 and 34 are spaced apart from each other over a relatively long distance, there is a difficulty in assembly and also in adjustments. In particular, there is a possibility that the tracking signal detecting light-receiving device 32 is located in a direction vertical to the plane of the drawing, in which case difficulty is increased even more.